Process for monitoring the quantization quality in digital radio or television transmission systems

ABSTRACT

In order to monitor the transmission quality of digitized signals, the quantization interference power is measured in a region where no useful signal is being transmitted. The quantization step for an associated D/A converter can be determined from the quantization interference power, and an assessment can be made, based on a comparison, as to whether the D/A converter is meeting the required quality parameters.

FIELD OF THE INVENTION

The present invention relates to a method for monitoring a transmissionquality of digitized signals that have been subjected to a quantizingoperation.

BACKGROUND INFORMATION

Reference signals, for example the test lines in a television image, canbe utilized to measure the signal-to-noise ratio of, for example, adigitized television image. This procedure requires, however, a complexand expensive test receiver with corresponding measurement technology.The monitoring of pilot signals requires a similar outlay.

A method for determining the quantization distortion of transmissiondevices is described in European Patent Application No. 0 030 383, wherea certain frequency band is provided at the transmitter end and fed intoa transmission device. At the receiver end, a level measurement isperformed within a transmitter-end frequency band, along with ameasurement outside that frequency band. The quantization noise ratio isdetermined from a difference between these two measurements.

A method for determining signal power and distortion power is describedin European Patent Application No. 0 069 972. Two signals are generated;the first digital signal is transmitted directly to a receiver end; thesecond signal is initially conveyed to a codec. The distortion power isdetermined from samples of the received signals. German PatentApplication No. 29 45 034 describes an arrangement for measuring a ratioof a signal distortion to a quantization distortion. Two level metersare used to determine the difference between a reference signal and anoise signal containing the quantization distortion.

SUMMARY OF THE INVENTION

According to a method of the present invention, digitized signals can beevaluated, in simple fashion and during transmission, in terms of theirtransmission quality. No interruption in service is required. It can beeasily evaluated as to whether a D/A converter provided in thetransmission path is operating properly.

The invention is based on the following realizations: The achievabletransmission quality of a digitized signal, e.g. the signal-to-noiseratio, is directly proportional to the resolution of an associated D/Aconverter located in the transmission path, and the maintenance of itsquality parameters. The present invention provides that in the uniformquantization of an analog signal during A/D conversion, given sufficientmodulation (generally >6 bits for a sine carrier or for multi-tonesignals such as those which occur with television distributionservices), the finite number of quantization steps results in an errorhaving a property of Gaussian white Gaussian noise. This white noise isuniformly distributed over the entire Nyquist spectrum, i.e. even in thefrequency regions in which no useful signal is being transmitted. Bymeasuring a mean of the quantization interference power in theseregions, the quantization step for the associated D/A converter can bedetermined and compared with an expected value. If there is animpermissibly large difference between the determined quantization stepand the expected value, an error signal is transmitted, for example viaremote control devices, to a central display.

Thus, a plurality of D/A converters, for example in a televisiondistribution network, can be monitored during operation in terms oftheir respective transmission quality. At the same time, data can begathered regarding the long-term stability of the D/A converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process diagram for a quality checking of a D/A converteraccording to the present invention.

FIG. 2 shows a location of the radio and television channels in abroad-band communications network.

DETAILED DESCRIPTION OF THE INVENTION

In a distribution network for radio and television signals, the radioand television signals are transmitted, for example, in a digitizedfrequency-multiplexed signal form on optical fiber cables GF and, at thesubscriber location TLN, converted by a fast D/A converter back into ananalog frequency-multiplexed signal form that can be processed bycommercially available radio and/or television receivers. An example ofa frequency-multiplexed signal is shown in FIG. 2.

Since the signal-to-noise ratio of the digitized transmitted signalsthat have been subjected to a quantizing operation is directlyproportional to the resolution of the D/A converter, quality checkingcan be confined to that parameter. As previously discussed, it isassumed that in the uniform quantization of an analog signal duringtransmission-end A/D conversion—ADC1 in FIG. 1—assuming sufficientmodulation (generally >6 bits for sine carriers or multi-tone signals),the finite number of quantization steps results in an error having theproperty of white noise. The general equation describing the averagequantization interference power or quantization noise power (lecturetext, Prof. Dr.Ing. W. Kaiser, {umlaut over (U)}bertragungstechnik II.,Transmission Technology II University of Stuttgart, 1982, pp. 229 to230) is:

P _(q)=(a ²/12),

where a corresponds to one quantization step.

The noise produced as a result, which is added to other noise sourcessuch as thermal noise, is uniformly distributed over the entire Nyquistspectrum, i.e. even at points where no useful signal is transmitted. Thequantization step a can be determined by measuring the mean quantizationinterference power, which of course must be greater than any other noisecomponents.

One point which is not occupied by the useful spectrum is the frequency10.7 MHz (or 5.5 MHz or 450 kHz). A sensitive, highly selectiveamplifier can be constructed at these frequencies, since suitablefilters are available. The following quantitative values apply to animplemented television distribution system. For a D/A converter with12-bit resolution and an output voltage of 1 Vss into 50 ohms, onequantization step corresponds to a voltage of is

(1Vss/2¹²)=244 uV

This corresponds to a mean quantization interference power

P _(q)=(a ²/12)Δ=4.9e ⁻⁹ W=−76 dBm.

This interference power is distributed over the entire Nyquistbandwidth, i.e. half the sampling frequency, or

(452 MHz/2)=226 MHz.

Since measurements are made only within a bandwidth of 300 kHz around10.7 MHz, 5.5 MHz, or 450 kHz, the expected interference power must becorrected by

10 log (300 kHz/226 MHz)=−28.7 dBm.

The expected interference power, referred to a bandwidth of 300 kHz, isthus

P _(q300)=−76 dBm−28.7 dBm=−104.7 dBm.

In the implemented distribution system, a degradation of the D/Aconverter to 8 bits should be detected. The expected interference powerof an 8-bit D/A converter is −104.7 dBm+4×6 dBm=−80.7 dBm. The outputsignal of the D/A converter DAV passes through a preselector VS to anamplifier V1 and then to a first ceramic filter KF1. Level preemphasisand adaptation to a second ceramic filter KF take place in amplifierstages V2 and V3. Following additional amplification V4, the noisesignal, with a level of +4 dBm, is applied to a rectifier and integratorGF. An A/D converter ADC2 digitizes the interference voltage and conveysit, via a corresponding interface, to a system controller for analysisA.

In an exemplary embodiment of the method according to the presentinvention, analysis can be performed by a comparator which compares thesignal being analyzed with an expected value determined for the D/Aconverter. In the exemplary embodiment, if a difference >=4 occurs, i.e.if the value falls below the expected resolution value a=12 by a valueof four or more, an error signal is prepared and displayed.

The present invention makes possible a simple and economical monitoringof the signal-to-noise ratio of a D/A converter during operation.Further possible advantages can be realized by replacing the individualamplifiers with commercially available IF amplifiers. The measurementfrequency can be changed by exchanging the ceramic filters. If avariable-frequency prefilter with oscillator and mixer stage is used,any desired region of the spectrum can be examined.

What is claimed is:
 1. A method for monitoring a transmission quality ofdigitized signals subjected to a quantizing operation, comprising thesteps of: measuring a quantization interference power in a predeterminedregion, wherein no useful signal is transmitted in the predeterminedregion; determining a quantization step for an associated D/A converteras a function of the quantization interference power; comparing thequantization step with an expected value; and providing an error signalif the quantization step deviates from the expected value by apredetermined difference.
 2. The method according to claim 1, wherein,when at least one of digitized radio signals and television signals aretransmitting and when the difference between the quantization step andthe expected value is at least four, the error signal corresponds to theexpected value.
 3. The method according to claim 1, wherein the step ofmeasuring the quantization interference power is performed in atelevision signal distribution network having frequency-multiplexedsignals with a frequency of 10.7 MHz.
 4. The method according to claim3, wherein the quantization interference power has a measurementbandwidth of 300 kHz.